For many years, Light Emitting Diodes (LED's) have provided an attractive alternative to traditional incandescent and fluorescent sources because of their small size and energy efficiency. In optical measurement systems, and other applications that benefit from consistent and/or predictable light sources, however, LED's have been slower to catch on. This is because the spectral output of an LED, both in terms of intensity and wavelength, varies greatly with temperature. Attempts have been made to control this variation, for example, by varying the current and/or voltage of LED's or even by heating the LED's prior to use. These methods, however, add additional complexity and expense and, for many applications, still fail to deliver an acceptable level of consistency.